1. Field of the Invention
The present invention relates to a display device, in particular, a display device including light-emitting elements arranged in active-matrix. In addition, the present invention relates to an electronic apparatus provided with the display device.
2. Description of the Related Art
In recent years, demands for thin displays mainly applied to TVs, PC monitors, mobile terminals, and the like have rapidly increased and further development has been promoted. The thin displays include a liquid crystal display device (LCD) and a display device equipped with a light-emitting element. In particular, an active matrix display using a light-emitting element is expected as a next-generation display for its features of high response speed, wide viewing angle, and the like in addition to advantages of a conventional LCD, such as thinness, lightness in weight, and high image quality and has been partially put in practical use.
The light-emitting element is also referred to as an organic light-emitting diode (OLED) and includes an anode, a cathode, and a layer which emits light (hereinafter referred to as an electroluminescent layer) when applying an electric field between the anode and the cathode, the layer including an organic compound or an inorganic compound between the anode and the cathode. An amount of a current flowing to the light-emitting element and luminance of the light-emitting element are in a predetermined relation, and the light-emitting element emits light at luminance corresponding to the amount of the current flowing to the electroluminescent layer.
However, a light-emitting element has a property that a resistance value (internal resistance) changes in accordance with an ambient temperature (hereinafter referred to as an environmental temperature). Specifically, with a room temperature set as a normal temperature, when the environmental temperature becomes higher than the normal temperature, the resistance value decreases. On the other hand, the resistance value increases when the environmental temperature becomes lower than the normal temperature. Accordingly, when the environmental temperature rises, luminance higher than desired is obtained as a current value increases. Conversely, when the environmental temperature decreases, luminance lower than desired is obtained as a current value decreases. Such a property of a light-emitting element is as shown in a graph indicating a relation between voltage-current characteristics of a light-emitting element and a temperature (see FIG. 30A). Further, a light-emitting element has a property that a current value thereof decreases with time. Such a property of a light-emitting element is as shown in a graph indicating a relation between voltage-current characteristics of a light-emitting element and time (see FIG. 30B).
Due to the properties of the light-emitting element, luminance thereof varies when an environment temperature changes and a change with time occurs. In view of the aforementioned conditions, there is proposed a display device in which an influence of variations in current value of a light-emitting element, which are caused by an environmental temperature change and a change with time, can be suppressed (for example, see Patent Document 1: Japanese Published Patent Application No: 2002-72964).
In the invention disclosed in Patent Document 1, a monitoring element and a light-emitting element provided in a pixel of a display portion are manufactured so as to have the same relation between a voltage applied between both electrodes and a current flowing to the light-emitting element under the same environmental temperature. However, as a pixel size of the display portion increases, the size of the monitoring element is also necessary to be increased; therefore, there is a problem of interrupting downsizing of a display panel.